Cryogenic deep-freezing device for products using a porous conveyor belt

ABSTRACT

The invention relates to a device for freezing articles, that comprises a conveyor with a porous belt maintained in position inside the device by drums, and a means for impregnating the belt of said conveyor with a cryogenic liquid, wherein the impregnation of the belt is carried out by partially or totally immersing the belt in a bath of cryogenic liquid, characterised in that it comprises a means for pressing the belt of the conveyor onto one of said drums.

The present invention relates to a cryogenic product deep-freezing process and device.

It is well known that certain food products are very difficult to deep-freeze and it is then necessary to use deep-freezing equipment with the following main requisites:

-   -   rapid deep-freezing with a machine occupying as small a floor         area as possible (smallest machine possible);     -   deep-freezing that has to be just as effective for the lower         surface of the product as for its upper surface;     -   the process must not mark the lower face of the products;     -   the products must not stick to the surface on which they have         been placed; and     -   where necessary, the equipment must allow only the surface of         the product to be deep-frozen, not the core thereof (and thus it         must produce what is called in the industry “crust freezing”).

The following products may be mentioned as examples: fish fillets covered with a marinade; pasty products (such as vegetable purees); meat portions covered with sauce.

At the present time, such very difficult products are treated in cryogenic tunnels by being immersed in liquid nitrogen, on narrow conveyors, if necessary providing orifices in the conveyor belt, to remove the gas formed and thus prevent the phenomenon of conveyor belt swelling/distortion, which would, as may be understood, cause productivity problems.

These immersion tunnels are well known for providing a high heat transfer over a short distance, and for being very compact (if transfer were not to take place by immersion, it would be necessary to have very long equipment for treating the same products).

In such cases of products that are very difficult to treat, it has also been demonstrated that the use of a cryogenic deep-freezer using a polymer (for example polyester) belt imbibed with liquid nitrogen or passing through a liquid nitrogen bath is particularly suitable. Such equipment is described in the document EP-576 665.

According to this prior technique, the size of the conveyor belt pores is such that cryogenic liquid can be retained therein, so as to ensure that the article is completely or partially frozen by heat transfer between the article and the cryogenic liquid retained in the porous support.

To carry out this operation successfully, the polyester belt on which the products to be crust-frozen are deposited must convey the products correctly through the deep-freezer, and in particular it is essential that it ensure a uniform transit time in the deep-freezer and in the liquid nitrogen bath. The expression “uniform transit time” is understood to mean a constant (i.e. substantially constant) transit time during a production phase, or over the course of a day, whatever the product treated, the batch treated, etc.

The second condition for the success of the operation is a uniform immersion in the liquid nitrogen bath, all the products being immersed in a substantially constant depth of liquid nitrogen.

For the reasons developed hereinbelow, the Applicant has demonstrated the fact that in certain situations, such a deep-freezer with immersion of a polymer belt in a cryogenic bath requires design improvements.

It should also be noted that some of these difficulties arise from the polymer nature of the conveyor (though this does have considerable advantages, as is known), these specific drawbacks not being observed with a metal belt. This is because a metal belt drive is “positive” (formed by systems of gearwheels that cannot slip) whereas in the case of a polymer belt it is necessary to use a smooth drum that drives a smooth belt.

Moreover, whereas a metal belt is perfectly rigid, a polymer belt will necessarily be subjected to deformations as it passes through the bath.

Having read the foregoing, it will now be clearer why the precision and the effectiveness of the process therefore lie in particular in controlling two points:

-   -   regularity of the transit time through the deep-freezer; and     -   regularity in the depth of the liquid nitrogen power encountered         by the product.

To meet the first condition, the belt speed must be steady and well controlled.

To meet the second condition, throughout the production period (for example throughout the day), the liquid nitrogen bath must have a substantially constant depth and the conveyor must pass completely flat through this bath, at a substantially constant distance from the surface of the bath. Thus, whatever the moment during the day, and irrespective of the position of the products, they always receive the same heat treatment since they are immersed in a substantially identical manner in the liquid nitrogen.

To give an example, if the conveyor is not considered to be completely flat, for example having a “bump” in the middle of the belt, the products that are immersed in the liquid nitrogen at this point will not be deep-frozen comparably to products passing through the bath along the edges of the conveyor belt, where this conveyor descends more deeply into the bath.

It will therefore be appreciated that the available machines according to the prior art cannot easily meet these two requirements and that very often the following two defects are observed:

-   -   slippage of the belt on its drive drum, hence a variable and         insufficiently controlled transit time;     -   distortion of the conveyor belt in the nitrogen immersion         tank—the belt no longer remains perfectly flat in the bath since         nitrogen gas bubbles form beneath the belt and swell it in the         central part thereof, whereas the edges remain in place at the         bottom of the tank. In addition, the tension in the belt also         has a tendency to raise the belt at its center, whereas the         edges remain in place in the bottom of the bath.

Among the existing solutions to the above-mentioned problems, mention may be made of the following arrangements in the prior art:

Regularity of the Transit Time/Regular Belt Speed:

In an attempt to provide a regular belt speed, certain manufacturers tension the belt by applying high forces. Thus, the phenomenon of the belt slipping on the drive drum is reputed to be less.

However, it should be pointed out that the higher the tension, the greater the phenomenon of belt distortion in the bath. In practice, a compromise must therefore be found between belt slippage and belt distortion, which means that the transit time and the bath depth are not sufficiently well controlled.

Liquid Nitrogen Bath Depth Regularity Problem:

To avoid the problem of belt distortion created by the high tension and to provide as regular a bath as possible, certain manufacturers reduce the width of the belt and the bath, or else divide the bath into several channels. Between the channels, what may be described as “skis” exert pressure on the belt in order for it to remain in position on the bottom of the tank.

As regards reducing the width of the belt, users of this type of machine, since they often require a high production capacity, which would mean wide machines (1000 mm in width), the solution consisting in reducing the width of the bath and of the belt is therefore not really satisfactory.

The use of “skis” for trying to limit the distortions of the belt divides the treatment zone into several channels. The user of the machine must then divide its production into as many lines with, in addition, the impossibility of treating products wider than the channels (maximum width: about 600 mm).

Moreover, in the absence of nitrogen, this system does admittedly divide the production into several channels; but when the tank is filled with liquid nitrogen, nitrogen gas bubbles form beneath the belt and create a pressure high enough to distort the belt and form two small bumps, one on each side of the “ski”.

Problem of Nitrogen Gas Bubble Formation:

To remedy belt swelling due to nitrogen gas bubbles trapped beneath the belt, certain manufacturers release the pressure created beneath the belt by perforating the latter. Thus, the gas can escape and the belt comes back down to a greater or lesser extent depending on the number and the size of the orifices.

However, the belt then no longer has a smooth surface and the products may sometimes be marked by the belt. In addition, this technique is not applicable to products smaller in size than that of the orifices (about 10 mm).

In every case, it has therefore been found that all the above solutions prevent the machine from being used to the optimum of its capabilities, while the quality and regularity of the deep-freezing of the product cannot be maintained at a high level.

As will be seen in greater detail below, the present invention provides a technical solution to the above problems, whereby it aims to establish a constant belt speed and a regular nitrogen bath above the belt at each point in the tank, by adopting a completely different strategy from those employed in the prior art, namely:

Transit Time Regularity/Regular Belt Speed:

To provide a constant belt speed, unlike the solutions of the prior art, the belt tension is set at as low a value as possible so as not to create a problem in the bath. The belt will therefore slip on its drive drum (the situation conventionally considered to be a draw-back). To obviate this phenomenon, rollers press the belt against the drum, thereby increasing the angle and the contact force between the drum and the belt. Locally, on the drum, the adhesion force is thus considerably increased without correspondingly distorting the belt in the bath, since the belt tension remains low in the rest of the machine. Thus, the transit time is perfectly controlled and is steady.

Thus, unlike the solutions of the prior art that recommend very high belt tensions, the present invention adopts a moderate tension, but locally presses the belt onto the drum by means of rollers, and the desired overall tension in the belt is obtained by moving the drums apart.

In other words, in order to try and better explain the phenomena below, the higher the tension in the polymer belt the firmer its attachment on the drum, but it then progresses with greater difficulty through the bath; conversely, the lower the tension, the “softer” the system, but it then does not turn easily—it “skates” over its drive drum.

The present invention applies a low tension to the conveyor—the system is therefore relatively “soft”; it therefore passes without any difficulty through the bath and this is compensated for by pressing the belt onto a single drum (which is equipped with the motor) via rollers (for example a free wheel that presses and follows the movement).

Regularity of the Liquid Nitrogen Bath Depth:

Thanks to the system described above, the belt naturally remains flat in the bath. This situation remains valid in the case of high-capacity machines with very wide belts (typically 1200 mm in width).

This is because the presence of the rollers allows the belt to be less tensioned in its entirety, and therefore the belt remains flat more easily (any tension creates distortion somewhere); to summarize, the presence of the rollers enables two technical problems to be solved at the same time.

The Question of Nitrogen Gas Bubbles:

As mentioned, when the bath is filled with liquid nitrogen, the nitrogen gas bubbles that form beneath the belt have a tendency to raise it and distort it.

The bubbles that form must therefore be removed without stoppage during production. To do this, according to the present invention, a space is provided between the belt and the bottom of the tank. This space allows the gas bubbles to escape via the entry and exit sides of the belt in the system, without creating an over-pressure beneath the belt and therefore without raising it. Thus, the belt remains in place, completely flat at a fixed distance from the surface of the liquid nitrogen bath.

In other words, the belt does not pass over the bottom of the tank, rather it passes at a certain “height” of a few mm or cm above the bottom of the tank (typically an order of magnitude of 1 to 10 cm is suitable for implementing the invention), but of course below the cryogenic liquid level, thereby providing a belt/bottom space. It is then conceivable, without at any time being tied to such an explanation, that the observed nondistortion is due to the fact that although the bubbles are directed toward the underside of the belt, they do not accumulate here—they naturally are discharged toward the entry/exit ends of the bath and the swashing observed by the Applicant at the entries/exits during its experiments confirm this hypothesis.

In conclusion, by applying this combination of techniques it is possible for the key points of the process to be completely controlled in those cases that are the hardest to treat:

-   -   the transit time through the apparatus and through the nitrogen         bath is completely controlled and constant;     -   the depth of the bath and the intensity of the cryogenic         treatment are completely controlled: the depth of the bath is         constant for all products—it can be easily adjusted depending on         the requirements.

It should also be noted that the proposed solution according to the present invention operates effectively, including with a wide belt, for example 1200 mm in width or more, thereby making it possible to achieve high production capacities. This also makes it possible to treat very wide products. However, of course it is also very suitable for smaller widths (300, 400, 600, 800 or 1000 mm).

Although an advantageous implementation has been described in which the two techniques—presence of rollers and space provided between the belt and the bottom of the tank—have been combined, it should be noted that, without departing from the scope of the present invention, it is conceivable that, for certain easier widths to treat, and also for certain products, only one of the technical features may be employed, in the presence of pressing rollers, while still obtaining the desired results.

In any case, thanks to the control provided by the process, the size of this type of machine may be increased and the production capacity is no longer limited by technical problems.

Moreover, the quality of the deep-freezing is constant and the process parameters may be adjusted to the best advantage, so as to optimize the process and reduce its cost.

Overall, it is the effectiveness and the efficiency of the process that benefit from the improvements proposed by the invention.

The present invention relates to a device for the deep-freezing of articles, comprising a porous belt conveyor held in place in the device by drums, and means for impregnating said belt of the conveyor with cryogenic liquid, the impregnation of the belt taking place entirely or partly by immersing the belt in a bath of cryogenic liquid, characterized in that it includes means for pressing the belt onto one of said drums.

According to one of the embodiments of the invention, said pressing means consist of a system of rollers fastened to a free wheel capable of pressing the belt onto said drum and of following the movement of this drum.

According to one of the preferred embodiments of the invention, a space is left between the conveyor and the bottom of the tank containing said bath, this space being suitable for enabling gas bubbles that form beneath the conveyor within the bath to completely or partly escape via the “entry” and “exit” sides of the conveyor of the device, without creating an overpressure beneath the conveyor.

Advantageously, said conveyor is made of a porous material, this being a fabric made of synthetic or natural polymer, whether woven or nonwoven, and preferably made of polyester.

The present invention will be better understood on reading the description of a nonlimiting exemplary embodiment, given solely for the purpose of illustration, and by reference to the appended FIGS. 1 and 2 which illustrate, in longitudinal and transverse views respectively, the structure of a deep-freezer having a polyester belt and a nitrogen bath according to the prior art and according to the present invention (employing a system of rollers and a space left between the belt and the bottom of the liquid nitrogen tank).

FIGS. 1 a and 1 b show schematically the main constituent components of a deep-freezer having a polyester belt and a nitrogen bath according to the prior art, as described in the introductory part of the present application.

Thus, the features of a preferred embodiment for implementing the present invention may be readily appreciated in FIGS. 2 a and 2 b:

-   -   the device comprises rollers for pressing the conveyor belt onto         the drive drum (the drum on the left in the figure, provided         with the motor), the rollers forming part of a free wheel         pressing the conveyor belt onto said drum and capable of         following the movement of this drum.

Also described below are measures for advantageously perfecting the implementation of the press rollers, which measures may prove to be advantageous in certain cases, especially according to the products treated or according to the type of conveyor belt of the user site in question. As will be understood, these improvement measures are not absolutely necessary for the present invention to be properly implemented—they may simply prove to be advantageous in certain very particular cases, namely:

-   i) When the belt is pinched/compressed between the drive drum and     the press rollers, a problem may sometimes arise at the moment when     the belt join fasteners pass over the drum—in some makes this join     is thicker than the belt (by way of illustration, about 5 mm as     opposed to 2 mm in the case of the belt) and may then remain jammed     at the press rollers, which may result in the conveyor belt being     immobilized.

It is then possible in this very particular case to fit the drum with bars that in no way impede its operation and make it possible to provide a passage for the thicker join. In some ways with such an arrangement, the following operation is observed:

-   -   the join arrives on the drum in a space between two bars. In         this case, the join may pass without any problem between the         drum and the rollers;     -   the join arrives on the drum just at a bar (join on the bar). In         this case, it cannot pass and the drum starts to slip on the         belt (the drum continues to rotate but the belt remains         stationary). Once the drum has slipped slightly, the join now         finds itself in a space between two bars and can then pass. This         small slip is very temporary and in no way impedes the correct         operation of the apparatus, and this improvement thus makes it         possible for the belt to continue advancing even when the join         passes between the drum and the rollers.

-   j) When the apparatus is operating cold, and according to the     features of the equipment in question, expansion may vary the     dimensions and the geometry of the apparatus very slightly. This is     sometimes sufficient to deflect the belt to one side, which would     have the consequence of rapidly damaging the edges of the belt and     therefore requiring it to be prematurely replaced.

In this particular case, a system for actively guiding the belt may therefore be proposed, in which a sensor detects the position of the belt.

As an example, when the belt is positioned too much to the right, the angle of the drive drum at the exit of the machine is automatically modified via an actuator, this having the consequence of repositioning the belt more to the left.

Conversely, when the belt is positioned too much to the left, the angle of the drive drum at the exit of the machine is automatically modified via an actuator, this having the consequence of repositioning the belt more to the right.

When the belt is thus correctly centered in the machine frame, the belt lifetime is very substantially increased. In addition, with the rubbing of the belt on the edges of the machine eliminated, the regularity of advance of the belt is further improved and the process may be even more precise.

Given below is an example of the implementation of the installation shown in FIG. 2.

Very good results are thus obtained with an apparatus measuring 6 meters in length and having a belt 1.2 meters in width, which deep-freezes a wide range of meat-based products, such as cooked hamburgers, raw or cooked meatballs, sausages, strips of meat, or cubes of ham. It will be understood that, for this wide range of products to be effectively treated, the deep-freezer with an immersion bath must be capable of being adapted. In certain cases, the transit time must be short (typically 1 minute) and the nitrogen bath very shallow (typically 5 mm in depth). For other, more difficult products, the transit time must be long (typically 10 minutes) and the bath must be relatively deep (typically 50 mm). Experience has shown that by applying the abovementioned technique, the entire range of products in question can be correctly treated, the deep-freezing being constant and regular over the course of time and during production phases. 

1-5. (canceled)
 6. A device for the deep-freezing of articles, the device comprising a porous conveyor belt held in place in the device by drums, means for pressing the belt onto one of said drums, and a bath of cryogenic liquid for entirely or partly immersing the belt to impregnate the belt with cryogenic liquid.
 7. The device as claimed in claim 6, characterized in that said pressing means comprises a system of rollers fastened to a free wheel capable of pressing the belt onto the drum and of following the movement of the drum.
 8. The device as claimed in claim 7, further comprising a space between the belt and a bottom of a tank containing the bath, the space being suitable for enabling gas bubbles that form beneath the belt within the bath to completely or partly escape via “entry” and “exit” sides of the belt, thus preventing an overpressure from being created beneath the belt.
 9. The device as claimed in claim 8, characterized in that said space has a height lying in the 1 cm-10 cm range.
 10. The device as claimed in claim 6, characterized in that said porous conveyor belt is made of a fabric made of synthetic or natural polymer.
 11. The device as claimed in claim 10, characterized in that said pressing means comprises a system of rollers fastened to a free wheel capable of pressing the belt onto the drum and of following the movement of the drum.
 12. The device as claimed in claim 11, further comprising a space between the belt and a bottom of a tank containing the bath, the space being suitable for enabling gas bubbles that form beneath the belt within the bath to completely or partly escape via “entry” and “exit” sides of the belt, thus preventing an overpressure from being created beneath the belt.
 13. The device as claimed in claim 12, characterized in that said space has a height lying in the 1 cm-10 cm range.
 14. The device as claimed in claim 10, characterized in that the fabric is polyester.
 15. The device as claimed in claim 14, characterized in that said pressing means comprises a system of rollers fastened to a free wheel capable of pressing the belt onto the drum and of following the movement of the drum.
 16. The device as claimed in claim 15, further comprising a space between the belt and a bottom of a tank containing the bath, the space being suitable for enabling gas bubbles that form beneath the belt within the bath to completely or partly escape via “entry” and “exit” sides of the belt, thus preventing an overpressure from being created beneath the belt.
 17. The device as claimed in claim 16, characterized in that said space has a height lying in the 1 cm-10 cm range. 